This project is concerned with the maintenance and application of analytical binding procedures for manuscript and characterization of gonadotropin receptor sites in the testis and ovary. Methods of gonadotropin labeling, characterization of the labeled tracer, and optimization of receptor assay methods, are continuously evaluated and improved. Computerized methods for analysis and binding data from kinetic and equilibrium experiments, plus corrections for the tracer characteristics, are applied to studies on the properties of gonadotropin receptors. Natives and modified gonadotropins are evaluated for receptor binding activity, for comparison with in vivo and in vitro biological activities. Particular applications have included the characterization of animal gonadotropins (such as rat LH) that sometimes show discrepant values of bioactivity and immunoactivity, and an analysis of the FSH-like activity of human chorionic gonadotropin.